System and method for over-the-air (OTA) testing to detect faulty elements in an active array antenna of an extremely high frequency (EHF) wireless communication device

ABSTRACT

Systems and methods for detecting faulty elements in an active planar antenna array of an extremely high frequency (EHF) wireless communication device. A planar antenna array having a matrix of dual-polarization modulated scattering probes is disposed within a near-field region of the antenna under test (AUT). Electromagnetic energy received from the AUT is converted to a complex electrical signal that is modulated by an electrical modulation signal and radiated as a scattering signal. The resulting electromagnetic scattering signal, received and converted to an electrical signal by another antenna, is used in a holographic image reconstruction operation via a backward-propagation transformation to reconstruct the signal spectrum radiated from the surface of the AUT. Configurable (e.g., electrically) scatter probes provide maximized modulation depths (MDs) over wide frequency ranges.

BACKGROUND

The present invention relates to over-the-air (OTA) testing of radiofrequency transceiver systems, and in particular to testing to detectfaulty elements in an active antenna array of an extremely highfrequency (EHF) wireless communication device.

As mobile wireless communication devices have become more widely usedfor many purposes, availability of sufficient signal bandwidth toaccommodate the many varied uses (e.g., streaming of video and/or moreuses of video in two-way communications in particular), has become acritical issue. This has led to more use of higher signal frequencies,such as extremely high frequency (EHF), which is the InternationalTelecommunication Union (ITU) designation for radio frequencies in theelectromagnetic spectrum band of 30-300 gigahertz (GHz), in which radiowaves have wavelengths of 10-1 millimeter, and are often referred to asmillimeter wave (mmW) signals.

For various reasons, including short line-of-sight signal paths due tohigh atmospheric attenuation, such devices often use active arrayantennas to beamform the signals to maximize signal path lengths (aswell as to better enable frequency reuse). As is known in the art, suchantenna structures include multiple active antenna elements, typicallyarranged in a regular array, e.g., a rectangular array of 16 or 25antenna elements (for radiation and reception of respectiveelectromagnetic signals) disposed in a 4×4 or 5×5 array, respectively.Accordingly, when testing such devices, it is important to be able totest each of the active antenna elements (e.g., all 16 or 25 antennaelements for a 4×4 or 5×5 array, respectively) to ensure compliance ofthe device to its design and/or performance specifications.

Current conventional testing techniques include performing far-field,compact range and near-field measurements of radiated energy from theactive antenna elements. The far-field method is often used for testingperformance of antennas that are generally used for communicationbetween two devices that are far apart, e.g., several λ, apart (where λ,is the wavelength of the carrier frequency of the radiated signal). Withthis method, the receiver, or range. antenna and the antenna under test(AUT) are separated by a range distance R of at least R=2D²/λ, apartfrom each other (where D is largest aperture dimension of the twoantennas). For an antenna with a large aperture (e.g., severalwavelengths in size), the range distance R can be large and dimensionsof the shielded test chamber using such a method will be large. Hence, Atest system using a far-field test method is undesirable for use in amanufacturing environment due to its size.

Further, while a far-field method may enable measuring of overallantenna performance and capturing the antenna radiation pattern, itcannot reliably detect defective elements in an antenna array since noreasonably detectable radiation difference would be observed whenmeasuring a fully active array with a minority of defective elements(e.g., a 5×5 element array with three of the 25 elements beingdefective). For example, using a single-point measurement of radiatedenergy steered in a broadside direction from such an antenna array doesnot reveal a significant difference (<1 dB) from that measured from anantenna array having no defective elements. Moreover, even if such asmall difference can be reasonably detected and measured, neither thenumber nor identities of the defective elements will be known, and evenwith no significant difference in measured performances of an antennaarray with defective elements when steering at broadside, performancedegradation may show up at other steering angles.

The compact range method, though similar in some respects to thefar-field method, differs in that an apparatus is used to transform aspherical wave into planar wave within a near-field region of the AUT,e.g., by using a reflector with a complex shape designed for suchpurpose. However, while the compact range method helps decrease the sizeof the required testing envelope (as well as the shielded test chamber),like the direct far-field method, this method still cannot detect andidentify faulty elements in an array in full active mode of operation.

Meanwhile, conventional near-field methods include near-fieldmeasurements that capture complex signals using planar, cylindrical orspherical scans, and simple coupling techniques that capture powermagnitude only. Near-field capturing of complex signals, generally inthe radiating near-field region, advantageously includes complex datathat can be mathematically transformed out to the far-field region toobtain far-field performance characteristics or transformed back to theantenna surface to help perform antenna diagnostics. While such systemsalso have smaller footprints than direct far-field and compact rangesystems, they generally use a single probe to perform a measurement scanusing a robot arm and, therefore, involve long test times to obtainmeasured data within the tested scanning surface (e.g., planar,cylindrical or spherical). While an electronic switched electronic arraymay be used in place of a mechanical device to accelerate themeasurement scan, when a large scan is needed the necessary large switcharray and design can be complex and expensive.

Simple near-field coupling techniques that capture power magnitude only,which tend to be simple and low cost and often used in manufacturingenvironments, use a coupler, or antenna, placed near the AUT to capturethe radiated power. A comparison power test with a measured power from areference, or known good, AUT is used to validate whether the AUT isdefective or not. In order to capture all potential defects, theaperture of the coupler needs to be as large as the AUT. For a small(e.g., 2×2) array, detecting which element is defective is not criticalso long as it can be determined whether the array as a whole isdefective or not. However, for an AUT with a large number of elements,design of a large coupler, essentially an antenna with a very largeaperture, though complex, is needed since near fields of all theelements must be measured to ensure accurate detection(s) of defectivearray elements. Further, such coupling method cannot identify individualdefective elements in a large array when under normal operation (fullyactive array). While such coupling method may nonetheless be used totest on an individual element-by-element basis to detect individualfaulty elements, this becomes increasingly time intensive and still doesnot enable testing of the array under normal (fully active) operation.

Traditional techniques use modulated scattering probes that offernon-optimal performance. Such probes are made of short dipoles, each ofwhich is connected to a respective diode. The length of each dipole isdictated by the area available and by how much coupling each probe haswith its neighboring probes, and thereby dictates, in turn, availablemodulation depth (MD) due to such fixed lengths that cannot later becontrolled or modified. In some cases, the available MD may be limitedto near-zero for some frequencies if not somehow designed otherwise.Even if a designed length is chosen to work well at one frequency range,it still cannot provide a good MD for a wider frequency range.

SUMMARY

Systems and methods are provided for detecting faulty elements in anactive planar antenna array of an extremely high frequency (EHF)wireless communication device. A planar antenna array having a matrix ofdual-polarization modulated scattering probes is disposed within anear-field region of the antenna under test (AUT). Electromagneticenergy received from the AUT is converted to a complex electrical signalthat is modulated by an electrical modulation signal and radiated as ascattering signal. The resulting electromagnetic scattering signal,received and converted to an electrical signal by another antenna, isused in a holographic image reconstruction operation via abackward-propagation transformation to reconstruct the signal spectrumradiated from the surface of the AUT. Configurable (e.g., electrically)scatter probes provide maximized modulation depths (MDs) over widefrequency ranges.

In accordance with an exemplary embodiment, an apparatus including anactive modulated scattering probe array, comprises:

-   -   a circuit board structure having a plurality of layers, wherein        the plurality of layers include alternating planar layers of        electrical conductors and at least one dielectric;    -   a first plurality of probe antenna elements disposed with a        first common orientation in a first array in a first one of the        plurality of layers;    -   a first plurality of electrical signal modulation circuits        connected among the first plurality of probe antenna elements        and disposed in a second one of the plurality of layers, wherein        each of the first plurality of electrical signal modulation        circuits comprises        -   first and second electrodes to receive a first bias voltage,        -   a first electrical signal modulation device connected            between the first and second electrodes and having a first            reactance in response to the first bias voltage, and        -   a first reactive circuit element connected between the first            and second electrodes and having a second reactance inverse            to the first reactance;    -   a first plurality of electrical impedances connected among the        first plurality of electrical signal modulation devices and        disposed in the second one of the plurality of layers;    -   a second plurality of probe antenna elements disposed with a        second common orientation in a second array in a third one of        the plurality of layers;    -   a second plurality of electrical signal modulation circuits        connected among the second plurality of probe antenna elements        and disposed in a fourth one of the plurality of layers, wherein        each of the second plurality of electrical signal modulation        circuits comprises        -   third and fourth electrodes to receive a second bias            voltage,        -   a second electrical signal modulation device connected            between the third and fourth electrodes and having a third            reactance in response to the second bias voltage, and        -   a second reactive circuit element connected between the            third and fourth electrodes and having a fourth reactance            inverse to the third reactance; and    -   a second plurality of electrical impedances connected among the        second plurality of electrical signal modulation devices and        disposed in the fourth one of the plurality of layers.

In accordance with another exemplary embodiment, a method for operatingan active modulated scattering probe array, comprises:

-   -   providing a circuit board structure having a plurality of        layers, wherein the plurality of layers include alternating        planar layers of electrical conductors and at least one        dielectric;    -   providing a first plurality of probe antenna elements disposed        with a first common orientation in a first array in a first one        of the plurality of layers;    -   providing a first plurality of electrical signal modulation        circuits connected among the first plurality of probe antenna        elements and disposed in a second one of the plurality of        layers, wherein each of the first plurality of electrical signal        modulation circuits comprises        -   first and second electrodes to receive a first bias voltage,        -   a first electrical signal modulation device connected            between the first and second electrodes and having a first            reactance in response to the first bias voltage, and        -   a first reactive circuit element connected between the first            and second electrodes and having a second reactance inverse            to the first reactance;    -   providing a first plurality of electrical impedances disposed in        the second one of the plurality of layers and connected among        the first plurality of electrical signal modulation devices;    -   providing a second plurality of probe antenna elements disposed        with a second common orientation in a second array in a third        one of the plurality of layers;    -   providing a second plurality of electrical signal modulation        circuits connected among the second plurality of probe antenna        elements and disposed in a fourth one of the plurality of        layers, wherein each of the second plurality of electrical        signal modulation circuits comprises        -   third and fourth electrodes to receive a second bias            voltage,        -   a second electrical signal modulation device connected            between the third and fourth electrodes and having a third            reactance in response to the second bias voltage, and        -   a second reactive circuit element connected between the            third and fourth electrodes and having a fourth reactance            inverse to the third reactance; and    -   providing a second plurality of electrical impedances disposed        in the fourth one of the plurality of layer and connected among        the second plurality of electrical signal modulation devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a switched probe array disposed proximate a DUT fortesting in accordance with exemplary embodiments.

FIG. 2 depicts implementation of a holographic image reconstruction viabackward-propagation transformation technique in accordance withexemplary embodiments.

FIG. 3 depicts capturing radiated near fields within a near-field regionof an AUT transformed back to the plane of the AUT in accordance withexemplary embodiments.

FIG. 4 depicts results of a differential holographicbackward-propagation transformation in accordance with exemplaryembodiments.

FIG. 5 depicts use of a switched probe array for re-configurablemultiple-coupling testing of multiple AUTs in accordance with exemplaryembodiments.

FIG. 6A depicts application of a modulation signal to an active switchedprobe array to produce modulated scattering signals in accordance withexemplary embodiments.

FIG. 6B depicts use of a passive switched probe array during parametricperformance testing in accordance with exemplary embodiments.

FIG. 7A depicts a testing environment for using an active modulatedscattering probe array near-field scanner in accordance with exemplaryembodiments.

FIG. 7B depicts a testing environment while using a passive scatteringprobe during parametric performance testing in accordance with exemplaryembodiments.

FIGS. 8A and 8B depict exemplary uses of diodes as modulation devices ofa modulated scattering probe array near-field scanner in accordance withexemplary embodiments.

FIG. 9 depicts a matrix configuration for providing electrical drivingsignals for modulation devices of a modulated scattering probe arraynear-field scanner in accordance with exemplary embodiments.

FIG. 10 depicts the matrix of FIG. 9 driven via switching circuitry inaccordance with exemplary embodiments.

FIG. 11 depicts generation of scattered and re-scattered modulatedscattering signals due to adverse probe radiation characteristics.

FIG. 12 depicts possible uses of inductive or resistive circuit elementsfor decoupling probes of a modulated scattering probe array fromincoming modulation signals.

FIG. 13 depicts a multi-layer substrate structure for a modulatedscattering probe array in accordance with exemplary embodiments.

FIG. 14 depicts relative horizontal and vertical positioning oforthogonal probes within a multi-layer modulated scattering probe arrayin accordance with exemplary embodiments.

FIGS. 15A and 15B depict modulated signal paths for orthogonal probeswithin a multi-layer modulated scattering probe array in accordance withexemplary embodiments.

FIG. 16 depicts test circuitry for monitoring performance of probeswithin a multi-layer modulated scattering probe array in accordance withexemplary embodiments.

FIG. 17 depicts a matrix for providing electrical driving signals viaswitching circuitry for modulation devices of a modulated scatteringprobe array near-field scanner with test circuitry for monitoringperformance of the probes in accordance with exemplary embodiments.

FIG. 18 depicts a modulated scattering probe including scatteringobjects, a varactor diode and an inductive load in accordance withexample embodiments.

FIG. 19 depicts a circuit model for the varactor diode and inductiveload of the modulated scattering probe of FIG. 18 in accordance withexample embodiments.

FIG. 20 depicts a circuit model for the scattering objects, varactordiode and inductive load of the modulated scattering probe of FIG. 18 inaccordance with example embodiments.

FIG. 21 depicts a plot of a scattered electric field strength versusfrequency for a modulated scattering probe having scattering objects, avaractor diode and an inductive load with example circuit elementimpedances (e.g., resistances, capacitances and inductances) inaccordance with example embodiments.

DETAILED DESCRIPTION

The following detailed description is of example embodiments of thepresently claimed invention with references to the accompanyingdrawings. Such description is intended to be illustrative and notlimiting with respect to the scope of the present invention. Suchembodiments are described in sufficient detail to enable one of ordinaryskill in the art to practice the subject invention, and it will beunderstood that other embodiments may be practiced with some variationswithout departing from the spirit or scope of the subject invention.

Throughout the present disclosure, absent a clear indication to thecontrary from the context, it will be understood that individual circuitelements as described may be singular or plural in number. For example,the terms “circuit” and “circuitry” may include either a singlecomponent or a plurality of components, which are either active and/orpassive and are connected or otherwise coupled together (e.g., as one ormore integrated circuit chips) to provide the described function.Additionally, the term “signal” may refer to one or more currents, oneor more voltages, or a data signal. Within the drawings, like or relatedelements will have like or related alpha, numeric or alphanumericdesignators. Further, while the present invention has been discussed inthe context of implementations using discrete electronic circuitry(preferably in the form of one or more integrated circuit chips), thefunctions of any part of such circuitry may alternatively be implementedusing one or more appropriately programmed processors, depending uponthe signal frequencies or data rates to be processed. Moreover, to theextent that the figures illustrate diagrams of the functional blocks ofvarious embodiments, the functional blocks are not necessarilyindicative of the division between hardware circuitry.

As discussed in more detail below, a test system with a small physicalfootprint may be designed to perform high speed measurement scans on adevice under test (DUT) during which all elements of its antenna arrayoperate in fully active modes (e.g., with all, or a desired subset, ofthe elements being excited simultaneously), as opposed to performingmeasurement scans on an element-by-element basis. Such testing may bemore advantageous when performed with a planar array antenna built aboveground as essentially a directional antenna with a large aperture, as aplanar scan is generally more appropriate than a cylindrical orspherical scan.

As also discussed in more detail below, such testing technique may beused to detect defective antenna elements in an array by performing aholographic image reconstruction of the electric fields (E-fields) atthe surface of the DUT antenna array using a backward-propagationtransform of the measured complex fields on a planar scan surface in thenear-field region of the antenna under test (AUT), e.g., with the testantenna array located at a distance of at least one wavelength (1λ) fromthe AUT to minimize perturbation on its radiated energy performancecharacteristics. The measured scanned E-fields received from the AUT maybe compared with previously measured scanned E-fields received from areference antenna array (RA) (e.g., a known good DUT with a similarantenna array). Differences between the scanned fields of the AUT and RAcan be used to determine whether and where defective antenna elementsare located within the AUT based on the backward-propagationtransformation.

More particularly, as discussed in more detail below, a large switchedarray may be used to capture the near-fields of the energy received fromthe AUT of the DUT. Such an array may include small probe antennas usinga modulated scattering technique, as opposed to conventional probeantennas. Further, such an array may also be used as a re-configurablecoupler, where the array searches and uses a subset of its probes thatcapture near-fields with predetermined minimum power levels and performa summation of the received power levels to compute a single power valuefor comparison with that of a reference DUT. Such a method may work wellfor small DUT arrays where, for purposes of the desired test,determining simply whether the DUT is defective or not is sufficient andknowing locations of defects is unnecessary.

Referring to FIG. 1, as introduced above and in accordance withexemplary embodiments, a DUT 12 having a planar antenna array 14 istested in a wireless test environment (e.g., within anelectromagnetically shielded enclosure forming, in accordance with knowntechniques, an anechoic chamber designed for the signal frequenciesbeing tested) in which the radiating and receiving antenna elements ofthe antenna array 14 define an AUT plane 14 a. A modulated-scatteringprobe array 16 (also planar) defines a plane disposed within thenear-field region Z₀ of the AUT 14 at a distance 18 of at least onewavelength (e.g., in terms of the nominal carrier frequency of thesignals of interest). As discussed in more detail below, complexnear-field signals from each probe of the modulated-scattering probearray 16 (e.g., designed to have dual polarizations along orthogonalaxes x, y of the of the array 16, thereby enabling measurements of bothmagnitudes and phases) are received by a test antenna (not shown) andmeasured, with the results then used to perform a backward-propagationtransformation 20.

Referring to FIG. 2, in accordance with exemplary embodiments, abackward-propagation transformation 20 (various forms of which are knownin the art) operates on the measured complex near-field signals first bymeasuring 22 near-field energy at the planar surface of probe array 16at the distance Z₀ from the AUT 14 of the DUT 12. The measured energy isthen processed in accordance with an Inverse Fast Fourier Transform(IFFT) 23 to produce a plane wave spectrum function 24 representing thereceived energy at each probe of the probe array 16. From this spectrumfunction respective probe compensation parameters are computed 25 toprovide probe compensation 26 for each probe of the probe array 16.These probe compensation parameters 26 are applied to the plane wavespectrum function 24 to compute a corresponding compensated plane wavespectrum function 28, that is then processed in accordance with a FastFourier Transform (FFT) 29 to determine the near-field energy 30radiated from the AUT 14.

Referring to FIG. 3, in an exemplary hypothetical test scenario, an AUT14 having a 4×4 array of 16 antenna elements (e.g., capacitive patchelements) defining the z=0 plane 14 a may be radiating its signal energysuch that maximum energy will be emitted from the regions shaded moredarkly red, minimum energy emitted from the regions shaded more darklyblue, and intermediate energy emitted from the regions shaded green andyellow. Similarly, the corresponding signal energy received by a probearray 16 having a 12×12 array of 144 dual-polarized elements (definingthe Z=Z₀ plane 16 a) has various energy levels indicated by variouslyshaded regions as discussed above.

Referring to FIG. 4, a holographic image reconstruction in accordancewith exemplary embodiments may use transformed near-field magnitudes atthe surface of an AUT 14 (e.g., a 4×4 antenna array) after transformingthe complex field difference captured at the scan surface 16 a onewavelength away from the AUT 14. For example, measured andback-transformed energy levels 14 r of a reference (e.g., known good)AUT may be compared to measured and back-transformed energy levels 14 bof a faulty AUT (with various energy levels indicated by variouslyshaded regions as discussed above) to produce a transform of the complexnear-field difference between the reference array 14 r and AUT 14 b toindicate a faulty AUT 14 m having faulty antenna elements 32 a, 32 b, 32c.

In other words, testing a fully-active AUT to identify potential defectsmay be described as follows. (As will be readily understood by one ofordinary skill in the art, even if an array antenna element isidentified as defective, it may still be possible (albeit perhaps onlyslightly) that it is not defective but instead circuitry and/orconnections within the DUT responsible for driving such identified arrayantenna element is/are defective. However, other electrical signaltesting of such circuitry and/or connections earlier in themanufacturing process can minimize likelihood of such non-antennadefects.)

First, perform measurements of complex near-fields (magnitude and phase)using the probe array 16 at the prescribed distance Z₀ of at least onewavelength from a reference (e.g., a known good) DUT operating in fullyactive mode. Such measurements will form a reference surface scan matrix14 r MREF for Z=0. Second, perform measurements of complex near-fieldsat the same distance Z₀ and same reference AUT position Z=0 for a DUToperating in fully active mode to form a measured surface scan 14 b MAUTfor Z=Z₀. Third, perform individual subtraction of the two matrices MREF(Z=Z₀)−MAUT (Z=Z₀)=MS (Z=Z₀) and transform the differential matrix MSdown to Z=Z₀ at the array antenna surface MS(Z=0) 14 d. The absolutevalue of the differential matrix |MS (Z=0)| shows the difference inE-field magnitudes 14 d at the surface of the AUT (Z=0) between thereference array results 14 r and AUT results 14 b. This test may beexecuted for any fully-active array state. For example, the arraytesting state may be set at a specific beamforming angle. If an elementof the AUT under test does not have the same phase delay or fieldmagnitude as in the reference array, the difference E-field at Z=0 willappear at the location of the defective element.

Referring to FIG. 5, similar testing may be done for multiple arrays ormultiple subsets of a larger array. For example, measured near-fielddata from the scan surface (Z=Z₀) can be used to perform data processingto test the arrays. A user may define a scan area for each array, or afull scan may be performed to detect probes within the probe arraydemonstrating significant near-field signal strengths with such probesthen used to define desired scan areas. This may enable the user tocreate re-configurable sub-apertures within a larger array apertureusing the large switched probe array 16. This may be particularly usefulwhen the AUT is small (e.g., a 2×2 array) and there is no significantadvantage (or perhaps even need) to know which element may be defective.(This may also be seen as similar to a coupling method using a singleantenna to capture radiated signals from the AUT.) All measured complexnear-field values from each probe of the modulated-scattering switchedprobe array within the chosen sub-aperture can be summed to give asingle measured value, which may be compared to that of a referencearray to test whether the AUT is defective (e.g., containing at leastone faulty element).

Referring to FIG. 6A, use of a modulated-scattering probe array in placeof a conventional probe antenna enables reductions in complexities of aswitched array design for capturing near fields at a specified scanningplane surface. As known in the art, modulated scattering may be achievedwith a frequency mixer over the air using a scattering object (e.g., anelectrical conductor) 19 connected to a non-linear circuit component(e.g., a diode) 17 driven by a modulating signal 15 m having amodulation frequency f_(M). When the scattering object 19 also receivesan incoming radiated signal 15 c having a carrier frequency f_(C) from atransmitter, frequency mixing occurs within the non-linear circuitcomponent 17, thereby producing modulation sidebands having lower f_(L)and upper f_(U) sideband frequencies below and above the carrierfrequency f_(C), respectively.

The resulting re-emitted electromagnetic signals 15 c, 15 ms togetherinclude the carrier frequency f_(C) along with the modulation sidebandshaving lower f_(C)−f_(M) and upper f_(C)+f_(M) sideband frequencies, andmay be captured by a test receiving antenna 34 for conversion to a radiofrequency (RF) electrical signal 35. The signal components at the lowerf_(C)−f_(M) and upper f_(C)+f_(M) sideband frequencies contain allnecessary electromagnetic information from the radiated signal wavehitting the scattering object 19 (e.g., amplitude and phase) that areneeded for purposes of the backward propagation transformation discussedabove. (As will be readily appreciated by one skilled in the art, if themodulation signal 15 m is non-linear (e.g., a square wave) then there-radiated signals will also include harmonic frequencies . . . ,f_(C)−3f_(M), f_(C)−2f_(M), f_(C)−f_(M), f_(C)+f_(M), f_(C)+2f_(M),f_(C)+3f_(M), . . . )

Such a modulated scattering technique offers multiple advantages. Forexample, near fields can be measured with minimum perturbation whensmall probes are used (e.g., as small as λ/6), whereas a conventionalantenna can introduce perturbation to the transmitting antenna (AUT)characteristics when disposed within its near-field region due to strongcoupling between the probes and antenna. Also, when used in an array,design of the resistive pair of wires going to the diodes of each probe(discussed in more detail below) that carry the lower frequencymodulated signals (f_(M)) is simpler than designing a conventionalswitched array receiving antennas with high-frequency combiners and/ormultiplexers.

Further, such a modulated scattering technique may be used in at leasttwo ways. For example, in a monostatic mode, a single antenna may beused for both the transmitting signals and receiving the resultingscattered signals. Alternatively, in a bistatic mode, signaltransmission and reception of resulting scattered signals may beperformed using a different dedicated antenna for each purpose.

Referring to FIG. 6B, in accordance with further exemplary embodiments,use of a scattering probe array in passive mode, i.e., with nomodulation applied, enables the system to be reused for parametric, orperformance, testing over the air. More particularly, the receivingantenna 34, located at a few wavelengths away from the AUT, may be usedas the test antenna while the scattering probe array is operated in apassive mode (e.g., with no modulation).

As known in the art, parametric testing focuses on performanceparameters (e.g., frequency response flatness, adjacent channel leakageratio (ACLR), error vector magnitude (EVM), receiver sensitivity, blockerror rate (BLER), etc.) that are not dependent upon antenna radiatedperformance. Method as discussed above can be used to perform thistesting so long as measurements are repeatable. Testing in a conductiveenvironment (e.g., via RF cable connections) generally provides the mostrepeatable measurement results. However, conductive testing cannot beused when RF signal ports are unavailable (e.g., devices designed toonly operate wirelessly). Hence, over-the-air (OTA) testing is necessarywith an additional requirement that measurements be repeatable and theOTA environment through which the signal travels (from transmitter toreceiver) has minimal effect(s) upon signal frequency response.

In a modulated-scattering test (MST) system, as discussed above, theprobe being modulated is physically small (e.g., no more than 0.25λ inlength) so as to minimize perturbation in performance of the AUT duringnear-field measurements. However, when the probe array is not modulated,the diodes are off (e.g., effectively appearing as open circuits) andthe probe array is effectively operating in a passive mode in whichsignal re-scattering at the nominal carrier frequency f_(C) isminimized. Hence, the probe array has minimal effect(s) on measurementsabout the nominal carrier frequency f_(C) the RX antenna. Moreimportantly, the probe array remains in a static position and mode,thereby introducing minimal measurement variations during parametrictesting.

Referring to FIG. 7A, in accordance with exemplary embodiments asdiscussed above, a complete test environment may include, withoutlimitation, the AUT 14, probe array 16 and test receiving antenna 34enclosed in an electromagnetically shielded enclosure 40 with internalsignal absorption materials (not shown), in accordance with knowntechniques, to form an anechoic chamber designed for the signalfrequencies being tested. Also included, typically external to theenclosure 40, may be RF signal interface circuitry 42, a controller 44and PC workstation 50. The controller 44 may provide one or more controlsignals 41 a to the probe array 16 for controlling scanning by theprobes (discussed in more detail below), and one or more control signals43 a to the RF signal interface circuitry 42 for controlling capturingof the RF signal 35 for conversion to an appropriate interface signal 43b for the PC workstation. The controller 44 may also communicate withthe PC workstation 50, e.g., to provide and/or receive controlinformation and/or data, via one or more signals 45.

Referring to FIG. 7B, in accordance with further exemplary embodimentsas discussed above, during parametric testing the probe array 16operates in a passive mode in which the probes 17 are effectively opencircuits (off) and re-scattering attributable to them is small comparedto the signal 15 c coming from the AUT. Accordingly, the dominant signalenergy arriving at the receiving antenna 34 is attributable to the AUTsignal 15 c about the nominal carrier frequency f_(C). During thisoperating mode, overall path loss can be measured between TX and RXsignals, and with proper design of its interior, the enclosure 40 willhave minimal effect(s) upon the measurements, thereby ensuring goodrepeatability of test results. Accordingly, parametric testing can beperformed with low degree of uncertainty when using the receivingantenna 34 to receive TX signals from the AUT (TX testing), as well astransmit RX signals to the AUT (RX testing).

Acts, modules, logic and method steps discussed herein may beimplemented (e.g., within and/or by the controller 44 and/or PCworkstation 50) in various forms, including, without limitation, one ormore computer programs or software code stored in one or more forms oftangible machine-readable media (e.g., memory) in communication with acontrol unit, including without limitation, a processor (e.g.,microprocessor, digital signal processor of the like) and memory, whichexecutes code to perform the described behavior(s), function(s),feature(s) and method(s). It will be readily understood by one ofordinary skill in the art that these operations, structural devices,acts, modules, logic and/or method steps may be implemented in software,firmware, special purpose digital logic and/or any combination thereofwithout deviating from the spirit and scope of the claims.

Referring to FIGS. 8A and 8B, in accordance with exemplary embodiments,a switched probe array may be implemented using one of at least twoforms of modulation: electrical or optical. For example, electricalmodulation may be applied via resistive wiring 47 r, 47 c to a PIN diode17 e connected between the associated probe elements 19. While thisnecessarily introduces metallic elements at or near the measuringprobes, which can introduce electromagnetic coupling between the probesof the array, with proper design (discussed in more detail below) suchcoupling can be minimized. Accordingly, minimal perturbation of the DUTradiated characteristics will be introduced, while also simplifyingprobe feed switching design and enabling low cost implementations.Alternatively, optical modulation may be applied (e.g., conveyed viaoptical fiber) via a modulated optical 15 mo (e.g., visible or infraredlight) or laser 15 ml signal to a photo diode 17 o connected between theassociated probe elements 19. This technique advantageously minimizesuse of metallic elements at or near the measuring probes, therebyminimizing potential perturbation of the DUT radiated characteristics.However, an array of numerous modulated probes would require equallynumerous optical fibers for which the design and placements and feedingof numerous optically isolated signals may be complex and costly.

With resistive wiring, or traces, the need for two traces for eachmodulated-scattering (MS) probe results in many traces required in anarray. For example, a 30×30 probe array with dual-polarization (tomaximize scattered energy) would require 1800 (30×30×2) probes, therebyrequiring 3600 (30×30×2×2) traces to be routed within the array. And, inaddition to the significant space needed to route so many traces, thereis the further need to minimize electromagnetic coupling among thetraces that may cause perturbations of the DUT radiated characteristicsas well as the scattering effects of the MS probes themselves.

As discussed in more detail below, in accordance with exemplaryembodiments, such unwanted coupling effects, costs and design complexitycan be reduced. For example, the number of resistive feed traces can bereduced to reduce unwanted coupling effects and design complexities inthe routing and switching of the feed traces. A dual-polarized probearray may be designed such that the horizontally-polarized (along thex-dimension) and vertically-polarized (along the y-dimension) probes areplaced at the same location (along the z-dimension) so that both signalpolarizations are captured at each probe location. Spacings among theprobe elements in the array should not be greater than a half wavelength(λ/2) to avoid unwanted fictitious effects after processing thetransforms of the measured values. Therefore, reducing the number offeed traces becomes even more important for operating an array scannerin the extremely high operating frequencies, such as at millimeter-wavefrequencies, as inter-element spacing becomes very small making routingof the feed traces becomes very challenging.

Referring to FIG. 9, in accordance with exemplary embodiments, a MSarray may be implemented as a matrix configuration of feed traces 47 r,47 c to the probes 17 for each polarization (horizontal and vertical) ofa n×n element array. During operation of this configuration, amodulation signal 15 m is applied to a column trace Ci 47 ci (e.g., oneof the column traces 47 c 1, 47 c 2, . . . , 47 cn) and a row trace Rj47 rj (e.g., one of the row traces 47 r 1, 47 r 2, 47 rn) is selectedfor connection to ground so that the modulation signal will only beapplied to a single probe (i, j). Hence, only one probe diode 17 ij willbe driven by the modulation signal 15 m while the remaining diodes aremaintained in a constant state, e.g., reverse biased by a applying apositive DC voltage+VDD via pull-up resistors 48 r to their cathodes andgrounding their anodes via pull-down resistors 48 c. Accordingly, for an×n array, instead of requiring 2×n×n feed, or control, lines, only 2nlines are required. For dual polarization probes the required linesbecomes 4n (2×2n).

Referring to FIG. 10, in accordance with exemplary embodiments, thenumber, complexity and routing of feed traces may be further reducedand/or simplified. For example, two routing circuits or systems 62 r, 62c (e.g., in the form of switching circuits or multiplexors) may be usedto connect the modulation signal M to a selected column line Ci (e.g.,via a column multiplexor 62 c) and ground a selected row line Rj (e.g.,via a row multiplexor 62 r and diode 63). Selection of individual probediodes 17 rjci by the multiplexors 62 c, 62 r may be initiated byrespective sets of multiplex control signals A0, A1, . . . , Am, B0, B1,. . . , Bm. Accordingly, the number of control lines becomes reduced to2 m+1, where m is smallest integer that is greater than log 2(n): A0,A1, . . . , Am, B0, B1, . . . , Bm, and M. For example, a 30×30 probearray will require only 11 (2×5+1) control lines. Adding dualpolarization to this proposed configuration will only increase thenumber of lines by one, since the multiplex control signals A0, A1, . .. , Am, B0, B1, . . . , Bm may be reused for the second polarization.Then, only one additional line will be needed for directing themodulation signal M to the vertical probe polarization array (as thevertical probe polarization modulation signal MV) or the horizontalprobe polarization array (as the horizontal probe polarizationmodulation signal MH). Hence, this may ensure that only the selectedprobe diode 17 rjci of the selected polarization array (vertical orhorizontal) is modulated while the remaining probe diodes are held inreversed-bias states.

Referring to FIG. 11, it is important that the design and layout of theMS array 16 ensure that among all the scattering elements 19 within thearray 16 (the probes that re-radiate energy received from the AUT 14)the only active element radiating energy is the selected diode 17 beingmodulated. Any other metallic element, wire or trace connected to theprobe element 17, such as the control lines 47 r, 47 c, will not onlyaffect probe radiation characteristics but may also experiencemodulation effects from the diode 17. A potential adverse effect isre-scattering by the control lines 47 r, 47 c of electromagnetic energy15 mrs at the modulation sideband frequencies fc+fm, fc−fm. This mayintroduce errors to measurements since the control lines 47 r, 47 c maynot only capture and re-scatter near-fields that are not located at thespecified probe 19, but also degrade polarization discrimination betweenthe horizontal and vertical probes as well as coupling between adjacentprobes.

Referring to FIG. 12, decoupling the control lines 15 can be done byincluding resistors or inductors 55 to increase resistance or impedanceat the measuring frequency at the connection points 19 to be decoupled.Increasing resistance or impedance reduces current flow. Inductiveimpedance response is a function of frequency and may be designed toexhibit high impedance characteristics at certain microwave frequenciesand low impedance at low frequencies. However, while inductors may besuitable for decoupling components at microwave frequencies, inductordesign at higher frequencies (e.g., millimeter-wave frequencies of tensof GHz and higher), becomes more complex and may not give a usefuland/or consistent impedance response for decoupling purpose in thatfrequency range. Accordingly, using resistors to increase resistance asthe decoupling components at such higher frequencies may often bepreferred, since their resistances ideally remain substantially constantover wide frequency ranges. In any event, it is important that themodulation signal voltage is sufficiently high enough to ensure that thediode 17 operates in its forward bias region.

Referring to FIG. 13, as noted above, capturing the maximum amount ofenergy radiated from an antenna requires complex signal measurements intwo orthogonal linear polarizations (e.g., “horizontal” and “vertical”relative to one another). To implement such a dual-polarized probearray, each probe element will need two linear polarizations. A simpledesign includes two short dipoles oriented orthogonally to each other.Other designs may also be used so as long as polarization discriminationbetween two probes is good and coupling between neighboring probes in anarray is minimal. Preferably the radiated signals should be captured atthe same planar location for each pair of orthogonal dipoles, or atleast relatively close (electrically) to each other. As depicted here, adesign having a layout supported by multiple layers of a supportstructure (e.g., a four-layer printed circuit board, discussed in moredetail below) may satisfy such a dipole location requirement and besuitable for managing the control line routing of all elements of theresulting planar array.

For example, first and second layers may support elements for ahorizontal dipole and its control lines. More particularly, the firstlayer supports column 47 c and row 47 r control lines that feed themodulation signal, via decoupling elements 55 and feed lines 74 ha, 74hb to the associated modulation device 17. The horizontal dipoleelements 19 ha, 19 hb are supported by the second layer, and receive andtransform radiated energy (from the AUT) to a RF electrical signal thatis conveyed by plated through-holes (vias) 76 a, 76 b to the modulationdevice 17. The resulting modulated RF signal is re-conveyed by the vias76 a, 76 b back to the dipole elements 19 ha, 19 hb that, in turn,transform it to an electromagnetic signal to be radiated as acorresponding scattered signal having horizontal polarization.

Similarly, third and fourth layers may support elements for a verticaldipole and its control lines. More particularly, the fourth layersupports column 47 c and row 47 r control lines that feed the modulationsignal, via decoupling elements 55 and feed lines 74 va, 74 vb to theassociated modulation device 17. The vertical dipole elements 19 va, 19vb are supported by the third layer, and receive and transform radiatedenergy (from the AUT) to a RF electrical signal that is conveyed byplated through-holes (vias) 78 a, 78 b to the modulation device 17. Theresulting modulated RF signal is re-conveyed by the vias 78 a, 78 b backto the dipole elements 19 va, 19 vb that, in turn, transform it to anelectromagnetic signal to be radiated as a corresponding scatteredsignal having vertical polarization.

Referring to FIG. 14, in accordance with exemplary embodiments as notedabove, the support structure for the probe array may be a four-layerprinted circuit board (PCB) 70. In accordance with techniques known inthe art, such a PCB 70 may include four layers 72 a, 72 b, 72 c, 72 d ofpatterned electrical conductors separated and mutually electricallyisolated by three layers 71 a, 71 b, 71 c of electrically insulatingmaterial (e.g., dielectrics). Preferably, the gap filled by the middleinsulating layer 71 c between the middle opposing conductive layers 72b, 72 c (layers 2 and 3) may be electrically small (i.e., based upon itsdielectric constant, its physical thickness corresponds to a fraction ofa wavelength of the nominal frequency of the radiated energy to bereceived and scattered). Alternatively, a two-layer PCB design may alsobe used, in which case the horizontal dipole elements 19 ha, 19 hb andtheir associated decoupling elements 55, feed lines 74 ha, 74 hb andmodulation device 17 are on a shared first layer, and the verticaldipole elements 19 va, 19 vb and their associated decoupling elements55, feed lines 74 va, 74 vb and modulation device 17 are on a sharedsecond layer. Further, preferably, the PCB may be a rigid board with aminimum thickness of 1-1.5 mm. Accordingly, a two-layer PCB may performwell for microwave frequencies as the gap (board thickness) between thetwo short dipoles is electrically small, though within the millimeterwave frequency region, a four-layer board design may be more suitable.

Referring to FIGS. 15A and 15B, as discussed above, selection of anactive probe is achieved by selecting a row i and a column j as in amatrix configuration. To avoid feed and/or control traces needing tosomehow cross each other, trace routing makes use of multiple (e.g.,two) PCB layers to accomplish the necessary routing. Feed and/or controltraces where the signal enters may be on one layer, parallel to eachother, while associated return traces may be on the other layer andparallel to each other, but orthogonal to the feed and/or controltraces. For example, the modulation signal for a horizontal probe may beintroduced via a column control line 47 c on layer 1, and returnedthrough another via 76 c to a row control line 47 r on layer 4.Similarly, the modulation signal for a vertical probe may be introducedvia a column control line 47 c on layer 1 and through another via 78 c,and returned via a row control line 47 r on layer 4.

With so many individual components required to implement a scatteredprobe array, as discussed above, it would be significantly advantageousto be able to monitor and/or periodically test the operation of eachprobe (along with its associated elements). In accordance with exemplaryembodiments, this can be achieved by appropriate implementation of thecircuitry controlling the array. For example, following manufacture ofthe array, proper operation of each probe assembly can be tested. If adefect is encountered, the defective probe assembly may be reworked tomake any necessary repairs (e.g., replace a damaged diode and/ordecoupling device, etc.). An incomplete set of measured data points mayalso be processed using an interpolation algorithm, such as compressedsensing algorithm, to recover missing data points. Knowing which datapoints are missing, or which probes are not working properly, and usinga data point recovery algorithm to recover missing data points may beexpected to enable better measurements than handling data points thathave incorrect values because of unknown defects.

Referring to FIG. 16, in accordance with exemplary embodiments, testcircuitry 80 for testing operation of each probe assembly may beimplemented with an operational amplifier 82, multiple resistances 84and analog-to-digital (ADC) circuitry 86, interconnected substantiallyas shown. In accordance with known principles, the operational amplifier82 and resistances 84 a, 84 b forming voltage dividers at its invertingand non-inverting input terminals operate to measure the voltage acrossa series resistance 84 t through which probe diode 17 current 85 tflows. Based upon the known value of the series resistance 84 t, themeasured voltage 83 signal represents the diode 17 current and may beconverted to a corresponding digital signal 87 by the ADC circuitry 86.The most likely types of errors include an open circuit for a specificdiode (e.g., a component is not properly soldered to the PCB and notmaking contact), a diode is shorted, or a bias resistor is shorted. Inthe first instance, the measured current 85 t when the diode is selectedwill be lower than the nominal current, and in the second instance, themeasured current 85 t will be higher than the nominal current.

Referring to FIG. 17, the probe test circuitry 80 may be connected via acommon point of the feed circuitry for each polarization at the point Mwhere the modulation signal Vm 15 m is applied. During probe testing,this voltage Vm may be fixed at a constant voltage that is sufficientlyhigh to ensure that the diode 17 being tested is in a state of forwardbias. Individual probe diodes 17 may be individually and sequentiallytested by measuring the current 85 t through each diode 17 as they areindividually selected. The current through each diode 17 may bedetermined by measuring the voltage drop Vt across the resistor 84 t inseries with the feed trace and decoupling devices 55 r 1, 55 r 2 anddiode 17. Alternatively, multiple probes may be tested simultaneously byscanning all probes or subsets of multiple probes (e.g., full rows orcolumns of probes) using different respective modulation frequencies todrive the scanned probes.

Performance may be further improved by using a modulated scatteringprobe that can be reconfigured to operate in a wide range of frequencieswhile still maintaining an optimal scattered electric field. Exampleembodiments employ a minimized scattering antenna concept used in themodulated scattering technique to maximize the modulation depth (MD) foroptimal probe scattering performance. As well known in the art, MD isdefined as the amplitude ratio of power in a modulated signal to thepower in the carrier, or a ratio of signal amplitudes such as electricfields and is a dimensionless parameter that quantifies how well ascattering probe may modulate a scattered signal. A large MD isindicative of a large amount of re-radiation produced from the probe. Ina modulated scattering probe, the signal amplitude is the measuredscattered electric field of the two states of the probe loaded with adiode (forward- and reverse-biased states). An improved MD (i.e., anincreased signal amplitude ratio) may be achieved by either increasingthe amplitude for the state that has the larger signal amplitude ordecreasing the amplitude for the state that has the smaller signalamplitude. The latter approach method may be done using the concept ofminimum scattering.

Scattering modes include structural mode and antenna mode. Structuralmode relates to the structure, shape and material of the antenna.Structural mode scattering is an electromagnetic reflection ordiffraction phenomenon from a metallic object. Antenna mode scatteringis essentially a re-radiation of the antenna reflected energy associatedwith the antenna radiation performance and feed network. Exampleembodiments rely upon antenna mode scattering, as opposed to structuralmode which needs to be minimized by using a thin and small structure,such as a dipole probe.

When an incoming signal hits a probe, such as a dipole, an inducedcurrent is created in the probe with a current distribution dependentupon the shape of the probe. This induced current is complex (withmagnitude and phase values) and, in turn, in turn produces electricfield re-radiation with a scattered field proportional to the integralof the induced current distribution on the probe. A way to minimizescattering from a probe is to load the probe with an inductive reactance(e.g., with an inductor) and thereby create a phase reversal of theinduced current in the vicinity of the load. Accordingly, such a loadmay be determined so as to cause the integral of the current to approachand perhaps reach zero, and thereby cause minimal or no electric fieldto be scattered. As a result the probe effectively becomes electricallyinvisible.

This minimum scattering of a probe is a narrow-frequency bandphenomenon. Therefore the MD of the modulated probe is large only for anarrow-frequency band. If an application requires a modulated probe towork for a wide range of frequency, a single modulated probe loaded withan inductive reactance will not work efficiently for all frequencies andnot at all at some frequencies.

This proposed invention describes a probe loaded with a varactor diodefor modulation and an inductive reactance load, such as an inductor,parallel to the diode. The inductive reactance can also be created usinga trace with a defined length. In the reverse biased state, the diodecan be modelled as a capacitor with small capacitance. This capacitor inparallel with the loaded inductor creates a parallel LC circuit withresonant frequency where probe electrical invisibility occurs as afunction of the L and C values. Since the capacitance value of thevaractor diode in the reversed biased mode is a function of the reversebias voltage, we can use it to tune the resonant frequency by applyingdifferent bias voltage. Therefore, we now have a way to reconfigurewhere electrical invisibility occurs as a function of the diode biasvoltage.

When this reconfigurable modulated probe is used in an array, where eachprobe is modulated one at a time while the others are set toreverse-biased states, we can ensure that the unmodulated probes are notperturbing the measurements as they do not scatter: no coupling effectwith the modulated probe and no perturbation of the carrier frequency.

Referring to FIG. 18, a modulated scattering probe including scatteringobjects, a varactor diode and an inductive load in accordance withexample embodiments may be modeled as a parallel LC (inductance L andcapacitance C) circuit 100 a in the form of a varactor diode 117 inseries with its associated scattering elements 119 a, 119 b plus aninductive load 155 ip in parallel. (Also included is a decouplingcapacitance 155 c to prevent a DC reverse bias voltage across the diode117 from being shorted by the inductance 155 ip) At the millimeter wavefrequencies of interest, a scattering probe 100 a with a variableresonant circuit may then be implemented by the diode 117 capacitance C(resulting when the diode 117 is reverse biased) in parallel with afixed inductor 155 ip (e.g., implemented as a lumped inductance or aconductive circuit trace designed to provide the desired inductance). Tovary the resonant frequency, the diode 117 capacitance C may be variedby applying different reverse bias voltage, e.g., via the probe elements119 a, 119 b via the point M where a modulation signal Vm is alsoapplied (FIG. 16). Once the needed amount of capacitance as a functionof the resonant frequency is characterized, one can set the reverse biasvoltage to produce the proper capacitance to interact with a predefinedfix inductive reactance to create an electrically invisible probe at aspecific frequency.

Referring to FIG. 19, a circuit signal model 100 b for the varactordiode 117 and inductive load 155 ip of the modulated scattering probe100 a of FIG. 18 in accordance with example embodiments includes thecapacitance Ct 117 c of the reverse-biased varactor diode 117 inparallel with an inductance 155 ip. (The decoupling capacitance 155 c isnot represented as it may be considered as a short circuit at thefrequencies of interest for purposes of scattering.)

Referring to FIG. 20, a more complete circuit signal model 100 c for thescattering objects 119 a, 119 b, varactor diode 117 and inductive load155 ip of the modulated scattering probe 100 a of FIG. 18 in accordancewith example embodiments includes the parallel LC circuit elements 117c, 155 ip plus impedances to be expected as part of the scatteringobjects 119 a, 119 b, e.g., serial inductances 155 isa, 155 sb (e.g.,inherent inductances of circuit traces forming the scattering objects119 a, 119 b) and resistances 155 ira, 155 rb (e.g., inherentresistances of circuit traces forming the scattering objects 119 a, 119b). Accordingly, as will be readily appreciated by those skilled in theart, the reverse bias voltage to be applied across the varactor diode117 may have a magnitude such that the diode capacitance Ct 117 c andparallel inductance 155 ip form a parallel resonant circuit, and mayhave another magnitude such that the diode capacitance Ct 117 c andseries inductances 155 isa, 155 isb form a series resonant circuit.Hence, as will be further appreciated, the former (parallel resonantcircuit) may present a high impedance and be effectively consideredelectrically invisible, and the latter (series resonant circuit) maypresent a low impedance and serve as an effective electrical conductorto allow maximum current flow for the modulating signal current.

Referring to FIG. 21, a plot of a simulation of scattered electric fieldstrength versus frequency for a modulated scattering probe havingscattering objects, a varactor diode and an inductive load with examplecircuit element impedances (e.g., resistances, capacitances andinductances) in accordance with example embodiments. Consistent with thediscussion above, this simulation shows that there are two distinctresonant frequencies, one where the effective circuit impedance is low(thereby enabling maximum current flow), similar to a series resonant LCcircuit, and one where the effective circuit impedance is high (therebyenabling minimal current flow), similar to a parallel resonant LCcircuit. (For this example, many others of which may be simulated withsimilar results, this plot shows simulated scattered field results as afunction of frequency using the loaded dipole model discussed above withan inductive load of 160 ohms (e.g., for an inductance L of 0.9 nH.)

Based on this discussion, it will be appreciated that, in addition tocontrolling how to route a modulated signal to a specified varactordiode (or probe) in a switched array (e.g., as presented in U.S. Pat.Nos. 10,536,226 and 10,659,175, the contents of which are incorporatedherein by reference) an additional parameter control may be implementedto control the DC voltage level of the modulated signal that is sent tothe varactor diode to control how much capacitance is presented. Theoptimal voltage level is frequency dependent and finding the optimallevel can be found sweeping it over the frequency range of interest.This “calibration data” corresponding to the optimal voltage level maybe stored in a table in memory within the test equipment controlling theswitched array (e.g., controller 44 in FIGS. 7A-7B). When a frequency isselected in the test equipment, the optimal modulated signal voltagelevel may also be selected. Implementation of such voltage levelselection may be designed using various voltage level control circuits,of which many are known in the art.

Accordingly, advantages realized with example embodiments includeachievement of an optimal modulation depth over a wide frequency rangeusing a reconfigurable design for tuning the described LC circuit ofprobe, varactor and inductor. Further, in a switched array, the longerprobes may be used without increasing coupling effects with neighboringprobes. Since typically only one probe is modulated and the others maybe in reversed biased states, they will be electrically invisible withminimal scattering and coupling effects. Further, increased dynamicrange may be achieved since at the collector antenna used to capture thecarrier and sideband signals the reference level of the tester should beequal to or larger than the level of the incoming signal, which is thatof the carrier signal. Dynamic range of the system is dictated by thatof the tester receiver and the smallest detectable signal is dependenton the set reference level. If the sideband signal level in the lowregion of the dynamic range for a strong carrier signal because themodulated scattering probe does not have good modulation depth, then thedynamic range for the sideband signals will be very limited. If thesideband signal level is near that of the carrier signal, then dynamicrange will be larger.

Various other modifications and alternatives in the structure and methodof operation of this invention will be apparent to those skilled in theart without departing from the scope and the spirit of the invention.Although the invention has been described in connection with specificpreferred embodiments, it should be understood that the invention asclaimed should not be unduly limited to such specific embodiments. It isintended that the following claims define the scope of the presentinvention and that structures and methods within the scope of theseclaims and their equivalents be covered thereby.

What is claimed is:
 1. An apparatus including an active modulatedscattering probe array, comprising: a circuit board structure having aplurality of layers, wherein said plurality of layers includealternating planar layers of electrical conductors and at least onedielectric; a first plurality of probe antenna elements disposed with afirst common orientation in a first array in a first one of saidplurality of layers; a first plurality of electrical signal modulationcircuits connected among said first plurality of probe antenna elementsand disposed in a second one of said plurality of layers, wherein eachof said first plurality of electrical signal modulation circuitscomprises: first and second electrodes to receive a first bias voltage,a first electrical signal modulation device connected between said firstand second electrodes and having a first reactance in response to saidfirst bias voltage, and a first reactive circuit element connectedbetween said first and second electrodes and having a second reactanceinverse to said first reactance; a first plurality of electricalimpedances connected among said first plurality of electrical signalmodulation circuits and disposed in said second one of said plurality oflayers; a second plurality of probe antenna elements disposed with asecond common orientation in a second array in a third one of saidplurality of layers; a second plurality of electrical signal modulationcircuits connected among said second plurality of probe antenna elementsand disposed in a fourth one of said plurality of layers, wherein eachof said second plurality of electrical signal modulation circuitscomprises: third and fourth electrodes to receive a second bias voltage,a second electrical signal modulation device connected between saidthird and fourth electrodes and having a third reactance in response tosaid second bias voltage, and a second reactive circuit elementconnected between said third and fourth electrodes and having a fourthreactance inverse to said third reactance; and a second plurality ofelectrical impedances connected among said second plurality ofelectrical signal modulation circuits and disposed in said fourth one ofsaid plurality of layers.
 2. The apparatus of claim 1, wherein saidfirst plurality of probe antenna elements comprises a plurality ofmicrostrip traces.
 3. The apparatus of claim 1, wherein said firstplurality of electrical signal modulation circuits comprises a pluralityof diodes.
 4. The apparatus of claim 1, wherein said first plurality ofelectrical impedances comprises plurality of resistances.
 5. Theapparatus of claim 1, wherein said first plurality of electrical signalmodulation circuits are connected among said first plurality of probeantenna elements with a plurality of electrically conductive vias. 6.The apparatus of claim 1, further comprising current measurementcircuitry connected to said first plurality of electrical signalmodulation circuits and configured to measure a plurality of respectivecurrents conducted through each one of said first plurality ofelectrical signal modulation circuits.
 7. The apparatus of claim 1,wherein said first and second common orientations are mutuallyorthogonal.
 8. The apparatus of claim 1, wherein said first and secondarrays are mutually disposed such that respective ones of said secondplurality of probe antenna elements are disposed in locations that aremutually proximate to respective ones of said first plurality of probeantenna elements.
 9. A method for operating an active modulatedscattering probe array, comprising: providing a circuit board structurehaving a plurality of layers, wherein said plurality of layers includealternating planar layers of electrical conductors and at least onedielectric; providing a first plurality of probe antenna elementsdisposed with a first common orientation in a first array in a first oneof said plurality of layers; providing a first plurality of electricalsignal modulation circuits connected among said first plurality of probeantenna elements and disposed in a second one of said plurality oflayers, wherein each of said first plurality of electrical signalmodulation circuits comprises: first and second electrodes to receive afirst bias voltage, a first electrical signal modulation deviceconnected between said first and second electrodes and having a firstreactance in response to said first bias voltage, and a first reactivecircuit element connected between said first and second electrodes andhaving a second reactance inverse to said first reactance; providing afirst plurality of electrical impedances disposed in said second one ofsaid plurality of layers and connected among said first plurality ofelectrical signal modulation circuits; providing a second plurality ofprobe antenna elements disposed with a second common orientation in asecond array in a third one of said plurality of layers; providing asecond plurality of electrical signal modulation circuits connectedamong said second plurality of probe antenna elements and disposed in afourth one of said plurality of layers, wherein each of said secondplurality of electrical signal modulation circuits comprises: third andfourth electrodes to receive a second bias voltage, a second electricalsignal modulation device connected between said third and fourthelectrodes and having a third reactance in response to said second biasvoltage, and a second reactive circuit element connected between saidthird and fourth electrodes and having a fourth reactance inverse tosaid third reactance; and providing a second plurality of electricalimpedances disposed in said fourth one of said plurality of layer andconnected among said second plurality of electrical signal modulationcircuits.
 10. The method of claim 9, wherein said first plurality ofprobe antenna elements comprises a plurality of microstrip traces. 11.The method of claim 9, wherein said first plurality of electrical signalmodulation circuits comprises a plurality of diodes.
 12. The method ofclaim 9, wherein said first plurality of electrical impedances comprisesplurality of resistances.
 13. The method of claim 9, wherein said firstplurality of electrical signal modulation circuits are connected amongsaid first plurality of probe antenna elements with a plurality ofelectrically conductive vias.
 14. The method of claim 9, furthercomprising measuring a plurality of respective currents conductedthrough each one of said first plurality of electrical signal modulationcircuits.
 15. The method of claim 9, wherein said first and secondcommon orientations are mutually orthogonal.
 16. The method of claim 9,wherein said first and second arrays are mutually disposed such thatrespective ones of said second plurality of probe antenna elements aredisposed in locations that are mutually proximate to respective ones ofsaid first plurality of probe antenna elements.